Head-up displays for an automotive or aircraft environment have been in use for several years. A continual effort has been applied to reduce size, weight, cost, number of optical and electronic components, and, at the same time, to improve both brightness and contrast in these head-up displays. Typically, a head-up display (HUD) will project light from an imaging device, such as a cathode ray tube display or liquid crystal display, where an image is created in an intermediate plane, from where it further gets projected towards the windshield through either refractive or reflective projection optics.
In an attempt to reduce size/cost of the head-up display, a diffusion film with extra imaging optics, both placed between the display source and a reflective optical element, were introduced. Such approach indeed allowed for a decrease in size of the display source (e.g., LCD) to some extent, however, at the expense of both needed additional imaging optical elements placed between the display source and the diffuser, and the undesired increase in an optical path between the display source and windshield. Not only does this add cost, this adds either extra weight/size to an already over-constrained in dash-space in a vehicle or aircraft.
Another approach uses an infrared laser source and a non-linear crystal both for infrared light conversion to visible and for light diffusion purpose in the intermediate image plane. Non-linear crystals are both prohibitive cost-wise to be employed in an HUD and have very low (a few percent) conversion efficiency. The approach has practical limitations as to get conversion efficiencies from infrared to visible light of more than ten percent, using a typical non-linear crystal (KDP) with the length of the order of one centimeter, the infrared light intensities of the order of 10 MWt/cm2 are needed. To achieve that, a tight focusing of the laser beam on the crystal is required, which, in turn, superimposes a strict requirement on the value of the allowable deviation angle from the direction of phase-matching condition (i.e. a few milliradians).
Another approach uses a holographic element for imaging. However, this approach uses a complicated ‘real-time’ two-beam re-construction of the hologram imbedded into a windshield that would be difficult and costly to implement in practice.
What is needed is an apparatus is a display that employs a diffuser that brings about light diffusion into a needed cone with gain, without the requirement for extra imaging optical elements between the display source and the diffuser, to allow for the implementation of simpler and more compact optics.